"关于以下绘图代码，哪一项说法是正确的？ import matplotlib.pyplot as plt x = [0, 1, 2, 3] y1 = [0, 1, 4, 9] y2 = [0, 1, 2, 3] plt.plot(x, y1, label='square') plt.plot(x, y2, label='linear') plt.xlabel('x axis') plt.ylabel('y axis') plt.legend(loc='upper left') plt.title('Functions') plt.show()"